The vehicle stability assist device (VSA) is known as a control device for stabilizing the handling of a vehicle. See patent document 1 (Japanese patent No. 3214824). Such a device is capable of restoring a disturbed behavior of a vehicle to a normal behavior by automatically braking the right and left wheels or front and rear wheels and using the braking force as a balancing force. This is typically accomplished by feeding back a deviation of the actual vehicle motion from a reference vehicle motion. In particular, patent document 1 discloses how a steering input which a vehicle operator intentionally applies may be restricted according changes in a side slip angle velocity obtained from a lateral acceleration.
Also is proposed the rear wheel toe angle control device (RTC) which may be used as a four wheel steering device that stabilizes the motion of the vehicle by steering the rear wheels so as to reduce the side slip angle of the vehicle to zero or a prescribed value as disclosed in Japanese patent No. 3179271 (patent document 2). There are a variety of approaches that can be used for such a purpose, and one of them is based on the changing of a steering angle ratio between the front and rear wheels depending on the vehicle speed (opposite phase in a low speed range and same phase in a high speed range). Such a control process, be it an RTC or 4WS, basically relies on a feed forward control that minimizes a deviation of a yaw rate response of an actual vehicle model to a steering input from a yaw response of an ideal vehicle model to the same steering input.
Because the VSA produces a yaw moment by distributing a braking force to different wheels, in theory, it can perform its function even in an operating region where the side slip angle of a wheel has exceeded a limit value. However, as it cannot be tolerated to apply the brake too often in practical applications, the VSA is typically activated only when the vehicle is under an extreme condition. On the other hand, as the RTC produces a yaw moment by causing a small side slip angle to the rear wheels, it is suited to be used in a normal operating range. Therefore, a combination of the VSA and RTC can produce a vehicle motion stability control system that is effective over an entire operating range. However, as the VSA and RTC are based on feedback and feed forward control principles, respective, these two control devices may interfere with each other when they are activated at the same time, and it may cause an unstable behavior of the vehicle.